The safety of the Li-ion battery is known to be a critical issue due to its high energy density and the high flammability of most of its components. Here we report on observations of significant gassing in large commercial Li-ion pouch cells during end-of-life (EoL) storage at supposedly ideal storage temperature, as well as the possibility to remove the evolved gasses through electrochemical degassing.
One known safety hazard in Li-ion batteries is gas evolution. Significant gas evolution can lead to an overpressure build-up in the cell, causing significant cell expansion, venting or bursting in the worst case. Many studies with respect to gas formation in Li-ion batteries have focused on gassing during formation cycling (SEI formation), or gas evolution under extreme conditions (e.g., abuse tests, thermal runaway). The gas compositions and related gas formation mechanisms are consequently fairly well understood. Reports on gas evolution under “normal” operation or storage are scarce. The evolved gases and their compositions might be different compared to gases commonly released during a thermal event in the Li-ion battery.
This study is based on large-format pouch cells (>60 Ah) from a renowned Li-ion cell manufacturer, cycled to EoL between 60 and 80% remaining capacity (SoH). Already during cycling, the thickness of some of the pouch cells increased by up to 50% by the time they reached EoL. In addition to the swelling during cycling, more than a third of the tested cells started to evolve significant amounts of gas during EoL storage at 5 °C, i.e. after the cycling was finished and the cells had reached the lower remaining capacity. This gas evolution was further analyzed to scrutinize the causes of this gassing. The analysis revealed that more than 95 % of the gas within the cell was comprised of hydrogen, methane and ethane, with hydrogen being the largest contributor. Small amounts of electrolyte solvents (EMC and DMC) were found, as expected due to the low vapor pressure at low temperatures. CO and CO2 levels were found to be below 1 %. Correlations between cell thickness increase, SoH and methane:hydrogen ratio were observed.
Selected gassed cells were carefully cycled to check their functionality and estimate their remaining capacity. During this process we observed that at specific conditions it was possible to electrochemically consume the evolved gasses within the cell.